In the amplification of a multi-tone high frequency input signal such as in wireless communications applications, undesirable intermodulation frequencies are typically produced, resulting in distortion of the output signal. A conventional technique for reducing this intermodulation distortion (IMD) has been to employ an additional correction amplifier to provide correction signals at the same distortion frequencies as the intermodulation (IM) products. The correction signals are generally provided at phases that are 180.degree. opposite those of the corresponding IM products so that the IM products and correction signals can be applied to an output combiner where they are cancelled because of the vector summation of the signals in the combiner. The amplified output signal, which also flows through the output combiner, then consists substantially of the fundamental input signal frequencies, i.e., the multi-tone components of the input signal.
A conventional feed forward low distortion amplifier circuit is depicted in FIG. 1. With this circuit, correction signals are generated by first applying a multi-tone input signal to an input coupler or power divider 2 where it is split into two signals. One of these signals is applied through a phase shifter 3 to a main amplifier 4 that amplifies the multiple tones but in the process produces the undesirable distortion products (or IM products). Coupler 5 is then utilized at the output of main amplifier 4 to couple a portion of the amplified signal (fundamental plus IM products) towards an input port of a subtracting coupler 7. Meanwhile, the other split signal from input coupler 6 is delayed by a delay line 6 and applied to the other input port of coupler 7. When the fundamental frequency components at both inputs of coupler 7 are at properly selected power levels and 180.degree. out of phase, the fundamental frequencies cancel within coupler 7 so that the output of coupler 7 consists entirely of the IM products. These IM products are then applied to a phase shifter 8 and amplified by a correction amplifier 9. The correction amplifier output which consists entirely of the IM products, is applied to one input of an output combiner 13. The other input to output combiner 13 is the amplified signal output from main amplifier 4, which has been channeled through coupler 5 and delay line 11. By adjusting the gain of the correction amplifier as well as the phase shift of the correction signals, the IM products can be cancelled within the output combiner resulting in a substantially distortion free output signal.
An example of a low distortion amplifier which generally employs the above described technique is found in U.S. Pat. No. 5,304,945 entitled "Low-Distortion Feed-Forward Amplifier", and assigned to the assignee herein. The circuit in this patent employs two correction amplifiers and correction loops to further reduce distortion products. The techniques disclosed therein rely on the complete cancellation of the fundamental frequency components in a pair of couplers to produce the optimum correction signals.
U.S. Pat. No. 4,583,049 entitled "Feed-Forward Circuit" discloses a similar distortion product cancellation technique for a single tone input signal. Therein, the single tone input signal is split by an input coupler, with one of the coupler outputs applied to a main amplifier. Amplification at the main amplifier produces undesirable spurious signals on both sides of the carrier. These spurious signals are cancelled using essentially the same technique described above in reference to FIG. 1 for the multi-tone case, except that the phase shift vs. frequency is more accurately controlled in the various paths within the circuit.